docs: def is_dimensionless_standard_name(xml_tree, standard_name):
      '''
      Returns True if the units for the associated standard name are
      dimensionless.  Dimensionless standard names include those that have no
      units and units that are defined as constant units in the CF standard name
      table i.e. '1', or '1e-3'.
      '''
      # standard_name must be string, so if it is not, it is *wrong* by default
      if not isinstance(standard_name, basestring):
          return False
      found_standard_name = xml_tree.find(".//entry[@id='{}']".format(standard_name))
      if found_standard_name is not None:
          canonical_units = found_standard_name.find('canonical_units')
          # so far, standard name XML table includes
          # 1 and 1e-3 for constant units, but expanding to valid udunits
          # prefixes to be on the safe side
          # taken from CF Table 3.1 of valid UDUnits prefixes
          dimless_units = r'1(?:e-?(?:1|2|3|6|9|12|15|18|21|24))?$'
          return canonical_units is None or re.match(dimless_units,
                                                     canonical_units.text)
      # if the standard name is not found, assume we need units for the time being
      else:
          return False